16 BULLETIN 15 0, U. S. DEPARTMENT OF AGRICULTURE 
The lumber is open piled in the storage room and allowed to con- 
dition. The time required for conditioning depends upon the species, 
size, dryness, and method of piling the lumber as well as on the tem- 
perature and humidity in the room. A conditioning period of one 
week is beneficial. 
Veneer may be successfully dried in mechanical driers of various 
types, in dry kilns, or by air drying (SI). In drying veneer, the 
internal stresses do not usually cause difficulty. However, when 
internal stresses do develop they may cause wrinkling, checking, or 
honeycombing of the veneer. (PI. 3.) These defects are easily 
recognizable and can be avoided by improving the drying conditions. 
In plywood plants where veneer is cut it is customary to glue it 
immediately following the drying. For general purposes, veneer is 
in the proper condition for gluing if it is flat, free from obvious 
defects, and at a moisture content of 5 per cent or less. Veneer which 
has been shipped or kept in storage, however, is seldom perfectly 
flat or dry, so it is customary to reclry it just before gluing. This is 
generally done in hot plate driers in which the veneer remains 
between the plates until sufficiently dry and flat. In the best practice, 
the veneer is then placed in piles so that it will keep flat, will cool 
off before gluing, and will not reabsorb much moisture from the air. 
Fancy veneer, such as burl, crotch, or other cross-grained pieces 
(pi. 2. A), is more likely to wrinkle and check in redrying than 
straight-grained veneer. Drying on hot plates may ruin this kind 
of veneer, so it is commonly redried by being placed under pressure 
between thick boards which are dry and hot. These boards absorb 
part of the moisture from the veneer without damaging the sheets. 
In exceptional cases veneer is glued without first being dried to 
a low moisture content. Fresh-cut veneer from green logs may be 
glued with highly viscous water-resistant glue (^5). By thus avoid- 
ing the preliminary drying operation the production cost is reduced 
and much loss from shrinkage in drying, is avoided. On the other 
hand, there is considerable warping and face checking in drying such 
plywood and the finished product is usually of low grade. 
If water-resistent plywood for aircraft is glued at a moisture con- 
tent of 12 to 20 per cent, higher strength values are obtained when 
tested wet than when glued at a low moisture content (8). Since 
aircraft plywood is dried only to about 10 per cent moisture content, 
warping and checking are not serious. Wet veneer stains more than 
dry veneer. 
MACHINING LUMBER 
Careful machining of lumber is necessary in the preparation of 
wood for gluing. The wood surfaces should be machined smooth, 
even, and flat. In panel work the pieces should also be machined to 
a uniform thickess. It is customary to machine the wood just prior 
to gluing so that the surfaces will not have an opportunity to be- 
come distorted from subsequent moisture changes. Where the four 
sides of a piece are to be glued it is best to glue in two operations and 
machine just before each operation. 
Surfaces made by the saw are usually rougher and more uneven 
than those made by planers, jointers, or other machines equipped 
with cutter heads. For this reason sawed surfaces used for cabinet 
